The δ18O and δD values of gypsum crystallization water together with δ18O and δ34S of sulfates were used as reliable techniques to study source of sulfur and mode of gypsum formation in selected central Iraqi soils. Six representative pedons on different geologic units were studied. The slope of 3.2 for δ18O and δD plot of gypsum crystallization water showed that evaporation was the major process of gypsum deposition in the study area. The mean δ34S value of +17.58 ‰ showed that Cretaceous sea sulfate followed by Tertiary is the source of sulfur in studied soils. The heavier δ34S value (+17.58 ‰) of the study area compared to central Iran (+13.5 ‰) proved that gypsum in central Iraq soils has been formed in the later stage of evaporation and that Iraqi landforms were cut off from the Tethys seaway after central Iran was evolved.